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<h1><a name="SECTION00281300000000000000000">
26.13 <tt class="module">fpectl</tt> --
         Floating point exception control</a>
</h1>

<p>
<a name="module-fpectl"></a>  
<p class="availability">Availability: <span
 class="platform">Unix</span>.</p>

<p>
<span class="note"><b class="label">Note:</b>
The <tt class="module">fpectl</tt> module is not built by default, and its usage
      is discouraged and may be dangerous except in the hands of
      experts.  See also the section <a href="fpectl-limitations.html#fpectl-limitations">26.13.2</a> on
      limitations for more details.</span>

<p>
Most computers carry out floating point operations<a id='l2h-5311' xml:id='l2h-5311'></a>
in conformance with the so-called IEEE-754 standard.
On any real computer,
some floating point operations produce results that cannot
be expressed as a normal floating point value.
For example, try

<p>
<div class="verbatim"><pre>
&gt;&gt;&gt; import math
&gt;&gt;&gt; math.exp(1000)
inf
&gt;&gt;&gt; math.exp(1000) / math.exp(1000)
nan
</pre></div>

<p>
(The example above will work on many platforms.
DEC Alpha may be one exception.)
"Inf" is a special, non-numeric value in IEEE-754 that
stands for "infinity", and "nan" means "not a number."
Note that,
other than the non-numeric results,
nothing special happened when you asked Python
to carry out those calculations.
That is in fact the default behaviour prescribed in the IEEE-754 standard,
and if it works for you,
stop reading now.

<p>
In some circumstances,
it would be better to raise an exception and stop processing
at the point where the faulty operation was attempted.
The <tt class="module">fpectl</tt> module
is for use in that situation.
It provides control over floating point
units from several hardware manufacturers,
allowing the user to turn on the generation
of <tt class="constant">SIGFPE</tt> whenever any of the
IEEE-754 exceptions Division by Zero, Overflow, or
Invalid Operation occurs.
In tandem with a pair of wrapper macros that are inserted
into the C code comprising your python system,
<tt class="constant">SIGFPE</tt> is trapped and converted into the Python
<tt class="exception">FloatingPointError</tt> exception.

<p>
The <tt class="module">fpectl</tt> module defines the following functions and
may raise the given exception:

<p>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-5308' xml:id='l2h-5308' class="function">turnon_sigfpe</tt></b>(</nobr></td>
  <td><var></var>)</td></tr></table></dt>
<dd>
Turn on the generation of <tt class="constant">SIGFPE</tt>,
and set up an appropriate signal handler.
</dl>

<p>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-5309' xml:id='l2h-5309' class="function">turnoff_sigfpe</tt></b>(</nobr></td>
  <td><var></var>)</td></tr></table></dt>
<dd>
Reset default handling of floating point exceptions.
</dl>

<p>
<dl><dt><b><span class="typelabel">exception</span>&nbsp;<tt id='l2h-5310' xml:id='l2h-5310' class="exception">FloatingPointError</tt></b></dt>
<dd>
After <tt class="function">turnon_sigfpe()</tt> has been executed,
a floating point operation that raises one of the
IEEE-754 exceptions
Division by Zero, Overflow, or Invalid operation
will in turn raise this standard Python exception.
</dd></dl>

<p>

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<li><a href="fpectl-example.html">26.13.1 Example</a>
<li><a href="fpectl-limitations.html">26.13.2 Limitations and other considerations</a>
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